Ok so I just did their online chat on the website and they said that if you previously purchased a Betta, and it is still within the extended warranty period (in my case, 3 years), then you do get 20% off (10% with the 2y warranty) of the purchase of an SE Plus. I didn't see anything about a coupon code in my old emails, so you might have to do an online chat to get their support to give you one. They said they're going to email me a 20% code. It would be super rad if you could stack the coupons, but I doubt that will work.
So, spending $60 for the warranty gets you $40-something extra off a new unit. That seems like a dang good deal, especially considering you have to ship these things back for repairs every 17 mins lol.
edit: Just got my code via email and it brought the SE Plus down to $343 and it even gives a discount on the extended warranty ($60 --> $48). Sweet!